1. Field of the Invention
The invention refers to a procedure for the production of writing or drawing stencils, and particularly to a stencil plate consisting of plastic and a border edging of plastic bound to the stencil sheet.
2. Description of the Prior Art
It is already well known how to produce the stencil plate and the border edging of a writing or drawing stencil in one piece, through injection molding (DT Gbm 76 30 452 ). Considerable difficulties arise from this process, and these difficulties can be traced back to the fact that the material strength of the total stencil sheet produced varies greatly. During the injection molding process, the border edging, consisting of greater masses of material, cools off slower after the injection molding than does the stencil plate which has less material strength. This can lead to considerable tension and molding flaws in the stencil plate. Beyond that, the tension can cause distortion of the stencil place, requiring reworking of the drawing surfaces of the border edging to correct. This type of re-working is particularly necessary if the symbol grooves in the stencil plate are produced simultaneously with the injecting of the entire stencil, since only then is the necessary precision achieved when the stencil is reversed, i.e., the distances between the writing edge and the reference line for the symbol grooves are exactly the same.
Furthermore, another procedure has already been recognized (DT-GM 76 30 452 ) wherein border edging of plastic (instead of the usual metal border edging) is slipped onto a prepared stencil plate, and secured thereon, for example, by meshing with indentations or protrusions present in the stencil plate. In order to achieve the desired precision when reversing, it is necessary with this procedure to rework the injection molded plastic border edging, and occasionally the stencil must be redone, also.
In another recognized procedure (DT-AS 12 37 927 ), metal border edges are inserted into an injection mold and the stencil plate is then molded in plastic. The plastic fills up the spaces in the metal border edging, thus producing a bond between stencil plate and border edging. Not even considering the fact that the above mentioned recognized procedure is plagued by difficulties arising as a result of the differing rates of cooling previously discussed, stencil plates produced in that procedure have the further disadvantage of distorting as a result of tensions which can occur in use and arise out of the different temperature coefficients of the plastic in the stencil plate and the firmly attached border edging. Besides that, the finishing of such metal border edging is relatively costly.